Line amplifier for telephony purposes



April 16, 1940. G. H. BAST ET AL 2,197,029

LINE AMPLIFIER FOR TELEPHONY PURPOSES Filed Nov 22, 1954 3' k gZ SZZMPatented Apr. 16, 1940 UNITED TATES PATENT, OFFICE LINE AMPLIFIER FORTELEPHONY PURPOSES Gerardus Henricus Bast and Frederik HendrikStieltjes,- The Hague, Netherlands, assignors, by mesne assignments, toInternational Stand-' ard Electric Corporation, New-York, N. Y., a,

corporation of' Delaware I Application November 22, 1934." Serial No.754,332 7 In The Netherlands November29, 1933 -1 clai (01. 179-170) Theinvention, relates to communication line such a load as a communicationline, which shall have a higher amplificationthan a corresponding outputstage of known "type, which uses a triode, and shall at the same timehave an available power output at least asgreat as the correspondingstage using a triode.

Anotherspecific object is to provide an output,

stage, such as above,'in which the power required for heating thecathode and supplying potential to the anode and screen, is equalto orless than in the corresponding known type of output stage using atriode.

According to this invention, the above and:

- other objects are attained by the use of a pentode Y whose degree ofscreening is above a certain value, connected so as to work into a loadwhose impedance bears at least a four. to one ratio to certaincharacteristicsv of the tube.

-'..In the accompanying drawing, several em- .39. bodiments of theinvention are illustrated- Fig. 1 represents schematically" an amplifieraccording to our'invention applied to a system in which it is especiallyadvantageous. In this system the amplifier is connected in the junctionbetween a two-wire line and a four-wire line.-

Figs. 2 and 3 represent further special applications wherein singlestage amplifiers constructed according to the .principles of ourinvention are provided with negative back coupling.

s In order to more precisely define the above special relationshipswhich are an essential feature of the invention, it isnecessary to referto a number of partial derivatives which define the characteristics of apentode. Also,ifor;analyz- 4. 6 ing thev performance of an amplifier.output stage in accordance with our invention, andfor comparing it withthe performance of a corresponding known outputstage using a triode,certain other partial derivatives are useful. To avoid 9,- complexexpressions, all these partial derivatives will be denoted,hereinaftenhy convenient designations as follows: v

' s=partial derivative of anode current with respect to control grid-potential-all other electrode potentials being constant. With respectto the usual an-'- ode current-grid voltage curve, s is, the slope ofthe curve. This, derivative s is well knownas the mutual conducte anceof the'tube. s=second partial derivative of anode current with respectto control grid potential'all other electrode potentials being constant.With respect to the usual anode current-grid voltage curve, s"-

is the curvature. of the curve. l g=partial derivative of anodepotential with respect to control grid potentialiall other electrodepotentials as well as the anode current being constant. This de-'rivative g iswell. known as the amplification factor of the tube. 1R1=partial derivative of anode potential wit respect to anodecurrenteall other electrode potentials being constant. This derivativeR1 isgenerally known as the A. C. resistance or' internal resist ance ofthe tube.

K =partial derivative. of control .grid potential with respect to totalemission current-all "other. electrode potentials be. ing constant. Y

K partial derivative of anode potential with respect to total emissioncurrentall other electrode potentials being constant.

Ks =partial derivative of screen grid potential with respect to totalemission currentall other electrode potentials being constant. I

The amplifienaccording'to the invention is characterized in 'this, thatfor the power-valve" a pentode is used, wherein 'in the operating regionthe ratio ofthe small variation of the total emission current whichresults fromagiven.

small variationin the screen-grid voltage (other voltages beingconstant) to the small variation ofthe total emission currentwhichresults from the small variation in the anode voltage (other ratio maybe called the screeningratiofl voltages again being constant) is of theorder of or higher.

and whereby, (at least at those frequencies at which the greatest soundenergies occur) the load impedance is at least equal to four times theresistance represented by Ksg.

(2) Ru lKsg The ratio KG K measures the extent to which. the. screen.grid shields the anode. For convenience therefore this The detailedstructural designs whichmay be used to produce a screening ratio ofvmere'than 50 are well known. In fact pentodes whose screening ratio isfar above 50 have already been known;

although these tubes were not used as power pentodes but rather forradio frequency ampli fication where freedom from feedback is theprimary requirement.

It is assumed that the .maximum available, alternating current power isdetermined, by the maximum allowable percentage of harmonics. Moreoverwith line amplifiers this percentage is lower than with otherapplications, since in general in an amplified line more than oneamplifier occurs and all said amplifiers deliver the same power.

The maximum alternating current power of the triodes ordinarily employedin line amplifiers is not determined by a sudden limit (e. g., by the Iattainment of a zero value of v anode voltage or anode'current) butratherv the limit is reached before this point. Thisis due to the factthat the characteristics are curved. The most important harmonicpossesses-a frequency which is twice the fundamentalfrequency and isherein called the second harmonic.

' To-a close degree of approximationthe percentage of the secondharmonics may be represented by /W R, (3) h-eIOOp Ru-Ri:[ Ru

In this equationh is the percentage, W the delivered alternating currentpower, R1 the internal tube resistance as previously defined, Ru theexterior resistance seen from the anode, and p is given by the equation:I 3

-The amplification A, defined as the ratio of the voltage across thenormal telephoneimpedance (obtained on the secondary of an ideal outputtransformer transforming the said secondary into Ru at theprimary) tothe grid voltage is given by the equation: i

in which g is the amplification factor and s the mutual conductance ofthe triode as previouslydefined. I

As. far as the percentage of harmonics is concerned the following isobserved:

The factor indicates the influence of the smaller'current variationwhich in case of a higher resistance is triode used for line amplifiers;the screen grid is arranged in the place of the anode of the triode I(that is to say: gs pentode g triode) The ideal- I necessary forobtaining the same alternating current power, the factor indicates akind of anode reaction. V As far as the output power is concerned,however, it is seen that the amplification becomes maximum at Ru=R1iTherefore this is the normal relation for line amplifier triodes.

It can be seen from Equation 3 that by-increasing Ru beyond the abovementioned value (R1) the harmonics are reduced in a two-fold way. Since,however, the amplification decreases when Ru increases beyond R1 theabove is not applied.

Now let us consider a somewhat idealized pentode havingthe followingproperties: cathode and control grid have the same structure as in aization of the pentode consistsin this, that it is,

- assumedthatthe entire emission current goes to the anode and thatthereis .no anode-reaction whatever. Then with the same voltages (andheating'current power) thewsame currents and consequently the same(curved) Ia.V;, teristics are*obtained.

The percentage of harmonics roo /2% The amplification:

If both-of these tubes are operated into loads" characwhose-impedanceequalsaRr of the .triode,.then. the pentode producestwice-the-percentageoi:

harmonics with-the same alternatingcurrent power and. also givestwicethe amplification-of.

the triode.

In order toobtain anequally small percentage;

of. harmonics the, pentod'e; shouldbe. loaded .by:

fourtimes-theRrof theg triodeh Then. the am; plification. becomes: fourtimes, (viz.- twice two;,

own interiorresistance;- I

An. actualpentode, however, is. not like above assumed.- idealizedpentodeafori 1. The-anode.voltageadoes infiuencethe totalemiz-zsioncurrent.v

times). as high aswithsthetriode loaded by itsrf 2. The emission currentisdividedbetween the.

anode and thescreen grid. v 3, An" anode voltage variation more stronglyinfluences the division between anode and. screen grid than the totalemission current.

' Consideringonly the first of the above tioned practical limitations,if itis desired to realize the possible higherarnplification of'thepentode when the pentode is operating into as load whichis at least fourtimes R1 of the triode, it is necessary that p 1 1 1 a it This has beendone by making:

50 Considering now, the second of the above mentioned practicallimitations, this reduces the eifective .mutual conductance s. Thenecessity of maintaining a small anode reaction is, therefore, evengreater, in order that a high amplification may be obtained in spite ofthis reduction in mutual conductance.

Considering now the third of the above mentioned practical deviationsfrom ideal, in general one can express a small change in anode currentby the following equation:

' 2b(AV,,AV,,)+'c(AV,,) etc. in which Va and V represent the anodevoltage and the grid voltage, respectively.

With a triode very approximately With a pentode such is not the casebecause the total emission current is divided; between the anode and thescreen grid, and because the said division depends on the value of thevoltage a SR IE and also 'ga i and 2SR5 c A consequence of all theserelationships in a pentode is that by a right. choice of the proportionV (which amounts to a correct choice of the loading impedance) it may bearranged that of the linear factors the SAW; predominates by far,whereas the square factors balance one another, at least if Theresult ofthis isthat if the load impedance is equal to the interior resistancethe alternating current amplitude is a larger fraction of the normalanode-current than the alternating voltage amplitude isof thenorrnalanode voltage. In other words; if the alternating signal input wereincreased the limit of zero anode current would be encountered-prior tothe limit of zero anode voltage.

Consequently there is a suitable margin for increasing the loadingresistance'without reducing the amount-of power which can be handledbefore encountering a discontinuity; the same is true of thecorresponding pentode. This margin is still further increased inproportion as the maximum allowable alternating current power with thetriode is reduced relative to the total supplied direct current power; IThe above applies only to pentodes. With tetrocles the additionaldiscontinuity at Va =Vsg imposesan additional limit on power and forthis reason only pentodes are considered.

From the above analysis it appears that if the conditions according tothe invention are met, a line amplifier is obtained having a higheramplification as well as a smaller percentage of harmonies for'an equaldelivered alternating current power, in comparison with thecorresponding triode. put energy is concerned, pentodes connected inaccordance with our invention are definitely superior to wellmatchedtriodes, provided that the load impedance is practicallyconstant, and furthermore it can be seen that a higher amplification maybe obtained, which represents a considerable advantage for lineamplifiers. I

The higher amplification which is obtainable according to our inventionprovides an opportunity of decreasing'the transformation proportion ofthe input transformer so that a larger width of the band and a betterquality are obtained, because it is not necessary to make such high de-It is thus evident that-so far as the outmands upon thesaid'itransformer since the am-- plification obtained with the pentodeis very high.

If Ri Ru a simple arrangement may be made for the change from four-wireinto two-wireconnections, whereby the acting losses are compensated by ahigher amplification in the direc-' tion 4-wire to 2-wire. It is truethat then the maximum delivered power will decrease but in general themaximum output energy is. not required in points in which one does notpass again into a four-wire connection.

In Fig. l of the drawing suchan arrangement is illustrated.

In such an arrangement the incoming four-wire circuit is coupled overthe input transformer I to the grid circuit of the pentode 2, whereasthe outary winding 4 of an output transformer 5, the primary winding 6of which lies in the anode circuit of the pentode 2.

v The winding of a second transformer I, the

put two-wire circuit 3 is connected with the secondsecondary 8 of whichis connected to the balance impedance 9, is in serieswith the saidwinding. The two-wire conductor 3 and the balance 9 are connected inseries in such a manner that their free terminals I0, I l produce'adifference in voltage equal to zero if the line-impedance and thebalance-impedance are equal. The incoming four-wire branch I2 isconnected to these points.

Both transformers are of the type usual for ordinary one-way amplifiers.With a very large R1 in comparison with Ru the amplification between thegrid circuit and the two wire circuit will be the same, independent ofwhether the second transformer is or is not present. In this connectionit should be noted that the condition above specified ior'the load Ru(namely that it must be at least four times Ksg) does not in any wayprevent the load from; being small in comparison with Ri. The effect ofthe currents set up by the two-wire line through the valve impedance isso very small that it may. beneglected; so that for speech transmittedfrom the twowire line to the four-wire line the efiect, is practicallythe same as if terminals 3 were simply connected to terminals it inseries With balancing resistance 9. In this case Ru is equal to twicethe optimum output value (since the transformers 5 and 'l' are the sametype used for simple line amplifiers) so that the energy to be delivereddoes not reach fully the optimum value. Further in case of Ri Ru theload impedance maybe artificially increased in any frequency rangeswhich do not require the full maximum power output, thus permitting theattainment at certain frequencies of an amplification somewhat higherthan-the normal amplification. By this arrangement the unequalattenuations of the line may be compensated.

A further advantage is that the invention by providing an unusually highamplification offers an opportunity of reducing the high amplificationby the use of negative back-coupling, while still having as muchamplification as Would be given by an ordinary triode.

Consequently the variations in the amplification grade in question beingdue to voltage-variations, ageing of the valves and the like become muchI smaller which is important for the upkeep of lines having a pluralityof amplifiers, whereas the percentage of harmonics decreasesconsiderably. This method with an auxiliary amplifier has been proposedalready, but the use of pentodes makes it possible to yield a similarefiect without extra valves by very simple solutions of which Figure 2illustrates an embodiment.

In the grid circuit of the pentode 2 a resistance l3 is inserted betweencathode and earth, whereas the transformer is likewise connected toearth over a second resistance 26. The latter has been designed forcoupling back the alternating current, whereas the resistancel3:substantially sup:

plies the negative grid voltage.

Many variations of this example are possible.

For example the resistance [3 could be replaced by a choke-coil withshuntresistance', whereby the resistance I4 may be abandoned.

Figure 3 shows a somewhat modified arrangement. Here the resistance M isarranged between the resistance l3 and earth. The high resistance l6arranged parallel-to l4 and the condenser l5 provide theright grid bias.

It is also possible to make the back-coupling plification becomes of theorder of 125 to 3'75 and consequently four to twelve times that inamplifiers having a corresponding triode.

We claim:

In a communication system a communication,

path which includes a two-Wire line and a four- The degree ofamplification of the amplifier is in that case equal Y to S'Ru and fors=2-5 mA/V the degree of amwire line, having outgoing and incomingcircuits, a combined amplifier and. repeater connected so as to couplethe two-wire line with the four-wire line, comprising in combination apentode includ-- ing a cathode, a control grid, a screen grid, asuppressor grid and an anode, the screen grid being designed tosubstantially completely shield the anode, an input transformer forsupplying the control grid and cathode oi the pentode with voltagesderived from the four-wire circuit, two output transformers having theirprimaries con-.1-

nected in series with said anode, their secondaries being connected inseries and in opposition in the outgoing circuit of the four-wiresystem, the cir-" cult of the two-wire system being connectedto one ofthe secondaries, and a balance network being connected to the othersecondary.

GERARDUS HENRICUS BAST. FREDERIK HENDRIK, STIELTJES.

